Note: Descriptions are shown in the official language in which they were submitted.
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ABRASIVE TOOL HAVING A UNITARY ARBOR
( 1 ) Field of the Invention
The present invention relates generally to grinding tools and more
particularly to
grinding tools for use in edge grinding of sheet glass. Use of the grinding
wheel of this
invention may improve glass quality and reduce process downtime.
(2) Background Information
The use of diamond containing abrasive wheels to contour andlor chamfer the
edge of flat glass (also referred to herein as sheet glass), such as that used
in the
automotive, architectural, furniture, and appliance industries, is well known
and is
typically carried out for both safety and cosmetic reasons. The abrasive
wheels of the
prior art include a profiled, bonded abrasive matrix disposed in a recess at
the periphery
of the wheel (see U.S. Patents 3,830,020 to Gomi and 4,457,113 to Miller).
During an
edge grinding operation, periodic reprofiling of the abrasive is typically
required to
produce consistent high quality glass. For optimum economic results it is
typically
desirable to minimize the downtime associated with reprofiling and to bring
newly
reprofiled wheels back on-line with minimal break-in and/or conditioning.
Therefore, there exists a need for a grinding tool and/or method for edge
grinding
of sheet glass that may provide for reduced downtime and/or improved grinding
performance.
One aspect of the present invention includes a grinding tool for shaping an
edge of
a glass sheet. The grinding tool includes an arbor and a wheel, the arbor and
wheel being
of unitary construction and having a conunon axis of rotation. The grinding
tool further
includes a recess extending along a periphery of the wheel with a bonded
abrasive
disposed therein. The bonded abrasive is sized and shaped for being prof led,
to shape an
edge of a glass sheet upon rotation of the tool about the axis. In one
variation of this
aspect the bonded abrasive may be further sized and shaped for being re-
profiled after
use.
In another aspect, this invention includes a method for shaping an edge of a
glass
sheet. The method includes providing a grinding tool as described in the
proceeding
paragraph, rotating the grinding tool about the axis, and applying the bonded
abrasive to
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the edge of the glass sheet. In one variation of this aspect, the method
further includes
reprofiling the bonded abrasive.
In still another aspect, this invention includes a method for profiling an
abrasive
matrix in a grinding tool. The method includes providing a grinding tool as
described in
the proceeding paragraph and machining a profile in an outer surface of the
bonded
abrasive matrix. In one variation of this aspect, the machining includes an
electro
discharge machining operation.
Figure lA is a schematic representation of a prior art grinding wheel;
Figure 1B is a schematic representation of a prior art grinding wheel;
Figure 2A is a cross sectional representation of one embodiment of a grinding
tool
according to the principles of the present invention;
Figure 2B is a cross sectional representation, on an enlarged scale, of a
portion of
the grinding tool of Fig. 2A;
Figure 3A is a view similar to that of Figure ZB, of another embodiment of a
grinding tool of this invention; and
Figure 3B is a view similar to that of Figures 2B and 3A, of still another
embodiment of a grinding tool of this invention.
Referring briefly to Fig. ZA, the present invention includes a grinding tool
that
may be useful in edge grinding a workpiece such as sheet glass for use in
various
applications, including automotive windows, architectural applications,
furniture, and
appliances. The grinding tool of this invention includes an arbor and an
abrasive wheel
having a unitary construction, i.e., an abrasive wheel in which the arbor is
an integral part
thereof. In one embodiment, grinding tool 100 typically includes a wheel
portion 110
having a body 120 with a recess 125 extending circumferentially along a
periphery
thereof. A bonded abrasive 130, i.e., a plurality of abrasive grains disposed
in a
framework of bond material, is disposed in the recess 125. Grinding tool 100
further
includes an arbor portion 150 integral with the wheel portion 110, i.e.,
integral with body
120. Arbor portion 150 may include a threaded end-portion 160 or other means
for
coupling to a conventional grinding machine (not shown).
The grinding tool of the present invention may advantageously provide for
improved quality grinding, and in particular reduced edge chipping, during
edge grinding
of sheet glass. Embodiments of this invention may also provide economic
advantages
such as reduced downtime during reprofiling, reduced power consumption, and/or
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reduced capital requirements. These and other advantages of this invention
will become
evident in light of the following discussion of various embodiments thereof.
As used herein the term arbor refers to a device coupled to the spindle or
axle of a
machine, and to wluch a tool such as a cutting, grinding, or polishing wheel
is mounted
for imparting rotary motion thereto. A unitary arbor refers to an arbor that
is an integral
part of the tool, i.e., in which a grinding wheel and arbor are of a unitary
construction.
The term edge grinding refers to a grinding operation in wluch a work piece,
such as
sheet glass, is shaped (e.g., contoured and/or chamfered) by grinding the edge
thereof.
Referring now to Figs. lA-2, prior art and the apparatus and method of the
present
invention are described. Figs. lA and 1B, illustrate examples of conventional
grinding
tools 50, 50', which typically include a grinding wheel 20, 20' mountable
(e.g., by
bolting) on an arbor 30, 30'. The grinding wheel 20, 20' typically includes a
bonded
abrasive 26 disposed thereon. Grinding wheels 20, 20' typically include a
flat, annular
body portion 22, 22' the periphery of which is radially inwardly slotted,
e.g., about the
center plane, to provide an annular recess 24, which holds and acts as a
support structure
for the bonded abrasive 26. The bonded abrasive 26 typically includes a U or V
shape
profile 28 ground therein, which is reproduced on the glass. Wheels of this
configuration
are commonly referred to as 'pencil edging' grinding wheels due to their
profile 28.
Grinding wheel 20, 20' is typically mounted to arbor 30, 30' through the use
of flange 40,
40', which serves to distribute operational stresses away from the central
hole.
As described briefly hereinabove, grinding tool 50, 50' is typically used to
shape
sheet glass such as that used in automobiles, furniture, architecture, and
appliances. The
grinding wheel 20, 20' is dressed periodically, e.g., with an aluminum oxide
abrasive
sticlc to re-expose the abrasive grains and remove any impacted glass fines
from the
surface of the wheel. When the profile 28 has worn sufficiently to be out of
tolerance, or
to produce edge chipping (edge chipping is often observed when the profile 28
becomes
attenuated), the wheel is removed and re-profiled by form grinding, e.g., with
a silicon
carbide wheel, or by electro discharge machining (EDM). During re-profiling,
the wheel
20, 20' is typically removed from the arbor 30, 30'.
The effort and downtime associated with removing the wheel 20, 20' from the
arbor 30, 30' for reprofiling purposes. is undesirable. Furthermore,
reengagement of the
reprofiled wheel with the sheet glass often results in initial edge chipping
of the glass.
While this problem tends to be transient in nature, i.e., self correcting with
time, sheet
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glass having edge chips must typically be scrapped at considerable expense.
This
problem tends to be significant since a typical wheel 20, 20' may be
reprofiled on average
from about 8 to 10 or more times during its useful life.
One solution to the problem, in particular for applications requiring
relatively high
edge quality, has been to grind scrap glass for some period of time after
reprofiling. This
approach, while it may reduce scrap, tends to significantly increase downtime
and reduce
the service life of the wheel.
One aspect of this invention is the realization that the above-described edge-
chipping problem may be related to run-out (e.g., an irregular or eccentric
path of rotation
by the grinding wheel) caused by imperfect concentricity between the arbor and
the
remounted wheel. Not wishing to be bound by a particular theory, it is
believed that
remounting the wheel to the arbor after reprofiling may result in slightly
imperfect
concentricity therebetween. As such the wheel operates essentially as though
it has not
been properly trued, i.e., rotating with a slight wobble. It is believed that
this "wobble"
causes the transient edge chipping problem until the bonded abrasive has been
sufficiently
worn.
One potential solution may be for the wheel to remain on the arbor during the
reprofiling process. This approach, while it may eliminate the transient edge
chipping
problem observed after reprofiling, would tend to be disadvantageous in that
it also
significantly increases downtime (by idling a grinding machine during the
reprofiling
operation) or requires glass grinding operations to maintain a relatively
large number of
relatively expensive arbors and therefore may significantly increase capital
costs and
operating expenses.
Referring now to Figure 2A, one embodiment of the grinding tool of the present
invention is illustrated. As described hereinabove, grinding tool 100
typically includes a
wheel portion 110 (i.e., a wheel means) having a body 120 with a recess 125
extending
along a periphery thereof. A bonded abrasive 130 is disposed in the recess
125.
Accordingly, bonded abrasive 130 functions as abrasive means and recess 125
functions
as support means for the abrasive. The bonded abrasive 130 typically includes
a profiled
grinding surface 128. In general it is desirable to size and shape the bonded
abrasive 130
to include sufficient depth in the radial direction to accommodate up to 10 or
more
reprofiling steps during the life of the grinding tool. The profile 128 is
typically U, V or
basket shaped but may include substantially any shape, including those
necessary to
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provide beveled, chamfered, Ogee, flat, arris, and the Iike edge patterns on
sheet glass. A
typical profile 128 varies depending on the glass thickness being ground and
may
typically be defined by a width (V~, depth (D), and radius of curvature (R),
as shown in
Fig. 2B. One standard profile that tends to provide a relatively long life and
satisfactory
edge quality is defined as follows:
W = 2 D(2R - D)
wherein width (Vii equals the glass thickness plus 0.5 millimeters and the
minimum radius of curvature (R) is approximately equal to the glass thickness
divided by
two.
For many applications a better surface finish may be achieved using a basket
profile in which:
W l 2 = RCos(a l 2) - (R - D)TafZ(a l 2)
wherein a is the included angle (between the frusto-conical edges of the
basket)
and typically ranges from about 50 to about 60 degrees. R is the radius of
curvature of the
bottom of the basket. V-shaped 128' and basket shaped 128" prof les are shown
in Figs.
3A and 3B, respectively.
Grinding tool 100 further includes an arbor portion 150 integral with the
wheel
portion 110, i.e., integral with body 120. Accordingly, arbor portion 150
functions as
axbor means for imparting rotary motion from a grinding machine to the wheel
portion.
Arbor portion 150 may include a threaded end-portion 160 or other means for
coupling to
a grinding machine. The arbor portion 150 and wheel portion 110 may be
fabricated from
substantially any material, e.g., an iron alloy such as tool steel, but are
typically fabricated
from a relatively lightweight material such as, but not limited to aluminum
alloys and
magnesium alloys. A relatively lightweight tool may advantageously reduce
power
consumption during use and result in less wear on drive spindles and other
grinding
machine components. A lightweight tool also tends to be relatively easy to
mount and
dismount from the grinding machine. A grinding tool including an aluminum body
with a
hardened steel insert at the mating face 165 between the grinding tool and the
grinding
machine may also be desirable in that it provides for a light-weight grinding
tool having a
highly wear resistant arbor portion 150.
Moreover, fabrication of these embodiments themselves may lead to cost savings
relative to the prior art. For example, the mutually engaging surfaces of both
conventional
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arbors 30,30' and grinding wheels 20, 20', should be manufactured to precise
tolerances
to help ensure that the mounted wheel runs true (i.e., concentrically) with
the arbor. By
fabricating the arbor and wheel in a unitary fashion, embodiments of the
present invention
eliminate the need for these close-tolerance fabrication steps, for potential
associated cost
savings.
Additional manufacturing cost savings may be realized due to potentially less
demanding design parameters associated with embodiments of this invention. A
single
conventional arbor 30, 30', is often used with tens, if not hundreds, of
grinding wheels.
Accordingly, such arbors are constructed robustly, to withstand the stresses
and wear and
tear associated with this long useful life. Contrariwise, the unitary
construction of the
present invention dictates that the arbor portion 150 is discarded along with
the wheel
portion 110, upon depletion of the abrasive matrix, for a shorter useful life.
As such, it
may be possible to fabricate these embodiments using less costly materials
and/or
construction techniques, without adversely affecting safety. Alternatively,
the arbor and
wheel portions (150 & 110) may be recycled by inserting new bonded abrasive
130 into
the wheel recess 125.
Grinding tool 100 may be substantially any size depending on the size and
shape
of the glass being ground. For typical applications, grinding tool 100
includes a wheel
portion 110 having a diameter ranging from about 75 to about 250 millimeters.
The bonded abrasive 130 may include substantially any abrasive grain material.
Conventional abrasives may include, but are not limited to, alumina, cerium
oxide, silica,
silicon carbide, zirconia-alumina, garnet, and emery in grit sizes ranging
from about 0.5
to about 5000 microns, preferably from about 2 to about 300 microns, and most
preferably from about 20 to about 200 microns. Superabrasive grains, including
but not
limited to diamond and cubic boron nitride (CBN), having substantially similar
grit sizes
as the conventional grains, may also be used. For most glass shaping
applications
diamond superabrasive grain is preferred. Edge quality tends to be determined
by the
diamond grain particle size. Increasing diamond grain particle size tends to
increase
grinding speed and wheel life at the expense of edge quality, while decreasing
diamond
grain size tends to improve edge quality at the expense of grinding speed and
wheel life.
One common superabrasive used for pencil edging automotive glass, includes a
particle
size distribution ranging from about 74 to about 88 microns (i.e., including
superabrasive
grains finer than U.S. Mesh (Standard Sieve) 170 and coaxser than U.S. Mesh
200). For
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chamfering, a common superabrasive abrasive includes a particle size
distribution ranging
from about 63 to about 74 microns (i.e., finer than U.S. Mesh 200 and coarser
than U.S.
Mesh 230). Architectural glass typically requires a finer finish than
automotive glass and
may be ground with two tools, e.g., a coarse tool having a superabrasive
particle size
ranging from about 125 to about 149 microns (i.e., finer than U.S. Mesh 120
and coarser
than U.S. Mesh 100) followed by a fme tool having a superabrasive particle
size ranging
from about 44 to 53 microns (i.e., finer than U.S. Mesh 325 and coarser than
U.S. Mesh
270). Superabrasive concentration within the bond matrix may vary relatively
widely, but
typically is in the range from about 8 to about 13 volume percent for
contouring
applications and about 12 to about 25 volume percent for chamfering
applications.
Increasing superabrasive concentration tends to increase wheel life and
decrease grinding
speed.
Substantially any type of bond material commonly used in the fabrication of
bonded abrasives may be used in the grinding tool of this invention. For
example,
metallic, organic, resinous, or vitrified bond (together with appropriate
curing agents if
necessary) may be used, with metallic bond being generally desirable.
Materials useful in
a metal bond matrix include, but are not limited to, bronze, copper, and zinc
alloys (e.g.,
brass), cobalt, iron, nickel, silver, aluminum, indium, antimony, titanium,
tungsten,
zirconium, and their alloys, and mixtures thereof. Bronze alloys with low-
level additions
of cobalt, iron, and/or tungsten are generally desirably for most glass edging
applications.
Softer, less wear-resistant bonds are typically used for furniture,
architecture, or appliance
glass and are generally made using relatively low levels of cobalt, iron,
and/or tungsten.
Increasing cobalt, iron, and/or tungsten at the expense of bronze tends to
increase wear
resistance. Automotive glass grinding applications typically utilize highly
wear resistant
bonds having relatively high levels of cobalt, iron, and/or tungsten since
long life is
preferred, to minimize wheel changes on fully automated lines and hence reduce
costly
downtime.
The grinding tool of this invention may be used with substantially any
conventional grinding machine, such as those provided by BYSTRONIC~ Machinen
Corporation (Switzerland), BANDO~ Chemical Industries Corporation (Japan), or
Glassline Corporation (Perrysburg, Ohio). During a typical grinding operation,
glass is
ground at rate ranging from about 2 to about 30 meters per minute. The
profiled abrasive
matrix may be dressed periodically using an implement such as an aluminum
oxide
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abrasive stick in order to maintain the grinding speed and edge quality. The
abrasive
matrix may also be reprofiled using conventional means, such as by form
grinding with a
silicon carbide wheel or by electro discharge machining.
The modifications to the various aspects of the present invention described
hereinabove are merely exemplary. It is understood that other modifications to
the
illustrative embodiments will readily occur to persons with ordinary skill in
the art. All
such modifications and variations are deemed to be witlun the scope and spirit
of the
present invention as defined by the accompanying claims.
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